In Ice, Mud and Blood I'm looking at what lessons we can learn from the past for future climate change. On page 99, I'm showing how large sea level changes can get. During the last ice age (yesterday, geologically speaking) there was a vast ice sheet over North America that would periodically collapse and fall into the North Atlantic, driving up the world's sea levels. Known as Heinrich events, these unsettling occurrences give us a real insight into what we can expect if the West Antarctic or Greenland ice sheets do disintegrate in a warming world.

From page 99:

Probably one of the best known places for ancient coral reefs is the Huon Peninsula on the eastern coast of Papua New Guinea. These fringing reefs have been worked on since the 1960s, most prominently by John Chappell at the Australian National University in Canberra. The Huon Peninsula is in one of those rather special parts of the world where the conditions are ideal for tropical coral reefs to grow, but at the same time is on the edge of a plate boundary. Stretching back at least 250,000 years, the Huon Peninsula has been rising to the tune of 3 metres per thousand years. The result is that any corals that grow on the submerged shoreline of the Peninsula eventually get thrust up into the air where they’re saved for a scientist to come fossicking about. Seen from above, the Peninsula look like an enormous set of vegetated steps; the fossil coral terraces appear to stretch away forever, preserving ancient lagoons and barriers that formed during rises in sea level.

John has undertaken a vast amount of work on the Huon Peninsula. Over the years, he has led a team that has measured, prodded and analyzed most of this ancient seascape. In 2002, he reported a study which showed that when a correction had been made for the rising land, the large reef terraces grew in sea levels that jumped 10 to 15 metres during Heinrich events; the deepening water gave the corals head-space to grow at pace with the rate of rise. After the sea level had shot up, the Huon Peninsula continued its inexorable rise, preserving a record of the big changes that had taken place. As the ice sheets subsequently reformed, the water was sucked back out of the ocean and the tide fell. Intriguingly, no sea level jumps were detected at the Huon Peninsula for the smaller stadials that happened between the Heinrich events. The collapse of the smaller ice sheets might have caused sea level changes of less than 3 metres; probably beyond what the coral reefs on the Huon Peninsula can detect. Worryingly, more recent reconstructions imply these estimates may be conservative. Work in the Red Sea suggests that sea level changes of up to 35 metres might have taken place during the Heinrich events. These are gargantuan changes.